Conventionally, electroless plating is broadly used as an industrial method for directly forming a film of a base metal, such as nickel (Ni), copper (Cu), or cobalt (Co), or a base metal alloy or a noble metal, such as silver (Ag), gold (Au), platinum (Pt), or palladium (Pd), or a noble metal alloy on a surface of a substrate. As the substrate for electroless plating, various compositions, such as metals, plastics, ceramics, organic compounds, and cellulose, can be used. Specifically, examples of the substrate include films of cellulose, fibroin, polymer resin such as polyester, cellulose triacetate (TAC), and so on; organic compound films of polyimide, polyethylene terephthalate (PET), polyaniline, photocurable resin, and so on; metal plates of copper, nickel, stainless steel, and so on; substrates of ceramics such as alumina, titania, silica, and silicon nitride, quartz glass, and so on; and ITO films. Among these substrates, a material having insulation properties and being difficult to deposit a plating film is usually immersed in a pretreatment solution such that a catalyst for electroless plating adheres to a desired portion of the insulating substrate.
As the catalyst for electroless plating contained in this pretreatment solution, a salt of a compound of a noble metal such as gold (Au), palladium (Pd), or platinum (Pt) or a salt of a compound of a base metal such as nickel (Ni) or tin (Sn) is used as metal ions in a pretreatment solution in many cases, but a method using colloid of a noble metal, such as gold (Au), is also known (Patent Literature 1).
The conventional pretreatment solution using noble metal colloid can form a catalytic nuclei of the noble metal colloid on a surface of an insulating substrate. However, in electroless plating, the catalytic nuclei have problems that the plating thickness varies compared to the catalytic nuclei of reduced noble metal ions in a pretreatment solution and that no uniform deposition is obtained. This is caused by that the adhesion to a substrate of catalytic nuclei of noble metal colloid is lower than that of catalytic nuclei of noble metal ions and that the catalytic activity of catalytic nuclei of noble metal colloid is lower than that of catalytic nuclei of reduced noble metal ions.
However, a method using metal ions has disadvantages such as that the number of treatment steps is increased and that the applicable electroless plating bath is limited. Accordingly, a procedure of reducing a noble metal salt in a pretreatment solution and allowing the formed noble metal colloidal particles to be adsorbed to a substrate has been proposed (Patent Literature 2).
However, the conventional noble metal colloid solution is readily affected by an acid or alkali and has problems that aggregation of nanoparticles in a noble metal colloid solution or desorption of the catalytic nuclei into the electroless plating causes abnormal deposition of a plating film and causes runaway of the electroless plating bath, resulting in breakage by used only once.